Differential temperature controls are widely used in many types of systems. In very recent times differential temperature controls for solar collector systems have become quite common. These types of systems normally include a temperature differential amplifier and control system that operate fans, pumps or similar equipment in a heating or cooling system. With the differential temperature controls, a resistance temperature sensor for the collector of a solar collector system, as well as a separate sensor for the storage temperature of the stored fluid are used. The two sensors normally are run by independent conductors to the solar collector and to the storage device.
Since solar collector type temperature control systems operate in widely varying types of climates, various types of limit functions are needed. In extremely hot climates, high limit controls are needed to prevent damage, and in the colder climates, protection is needed in the system when the storage fluid reaches or nears its freezing temperature. In the prior art type systems, solar collector temperature controls using resistance type sensors are common, but the limit functions needed are normally separate limit functions which use separate limit controls or are built into the differential amplifier itself. This makes the device rather inflexible and expensive.